The disclosure relates to a gamma voltage generator, and more particularly to a liquid crystal display (LCD) device comprising a gamma voltage generator.
A gamma voltage generator is used in active matrix liquid-crystal displays. The main function thereof is to provide a digital coded signal converter. With respect the characteristic curve of a liquid-crystal display, the input image data is adjusted properly along the curve. Through this conversion characteristic curve, the hue, gray level, contrast and color of the display can be adjusted.
FIG. 1a shows the relation of the voltages in a typical normally white mode liquid-crystal display (LCD) device to the display property (T) of a LCD device, where T is the transmittance. FIGS. 1b and 1c are a characteristic curve of image codes of a liquid-crystal display. To acquire the characteristic curve of FIGS. 1b and 1c, an adjusting mechanism is required for compensating the change of the property of the display due to external data input to the display. The adjusting mechanism is a gamma voltage generator. FIG. 1d shows a conversion curve of the data codes of the gamma voltage generator relative to the voltages.
In a Twisted-Nematic (TN) LCD, the characteristic curve of the transmittance of the liquid-crystal material to the voltage is a nonlinear curve. Therefore, in a gamma voltage generator, the greater the number of sampling nodes of the reference voltage, the smaller the approaching error of the characteristic curve can be obtained.
In the high resolution trend, for example, an 8-bit data driver can provide 256 gray levels, if an optimum adjustment to these 256 gray levels is desired, the adjustment is made through 256 externally provided reference voltage nodes. Further, the adjustment is performed one by one. However, the driving voltage of liquid-crystal material is alternative voltage, and therefore, each of the positive and negative polarities needs 256 reference voltages. Totally, 512 external input reference voltages are necessary for adjustment, but it is impractical to make so many inputs of the reference voltage in one driving IC. In fact, it is seldom to make such a work.
In general, only a few reference voltages are externally provided, and the driving IC, by a potential division method with a fixing ratio, the desired reference voltages are acquired by potential division without being provided externally. FIG. 2 is a schematic diagram of a conventional gamma voltage generator. A data driver of a LCD device generally requires a set of central symmetric gamma correction voltage. This central voltage is obtained from VCOM=(VCC+VGND)/2. The input voltages VCC and VGND pass through a gamma voltage generator 20 for voltage division so as to obtain a plurality of voltages to control the brightness of display.
A panel of the LCD device comprises a plurality of pixels. Each pixel includes three color sub-pixel units for displaying primary colors, that is, red, green, and blue. Brightness of three color sub-pixel units are controlled by voltage output from gamma voltage generator 20. Since three color sub-pixel units included in a pixel are controlled by the same voltage, each color pixel unit cannot be individually controlled. Therefore, the color image cannot be optimally adjusted.
FIG. 3 is a schematic diagram of another conventional gamma voltage generator for solving the above problem. Conventional gamma voltage generator 30 comprises resistor strings 32, 34, and 36. Resistor string 32 generates voltage to control the red color pixel. Resistor string 34 generates voltage to control the green color pixel. Resistor string 36 generates voltage to control the blue color pixel. Thus, the color image can be optimally calibrated as conventional gamma voltage generator 30 controls color pixel units. The sum of resistors of gamma voltage generator 30, however, is triple that of gamma voltage generator 20 such that cost and layout space of gamma voltage generator 30 are increased.